1. Field of the Invention:
The present invention relates to double containment pipe fittings and their assembly.
2. Description of the Prior Art:
The general concept of providing a double or dual containment pipe system wherein an inner carrier pipe is concentrically located within an outer containment pipe to deliver dangerous or hazardous fluids is well known and an accepted commercial practice. Historical applications for such systems have been found in the nuclear, gas petroleum production and refining and chemical processing industries. The inner pipe is used to transport the hazardous or toxic fluid while the outer pipe is present to confine any leaks. Thus, it is also known to provide the annulus between the concentric pipes with various types of detectors and/or drainage apparatus to handle leakage. Examples of double containment pipe assemblies are shown in U.S. Pat. Nos. 4,786,088; 4,886,305 and 4,930,544.
With the advent of stricter governmental regulation concerning the piping of petroleum products and hazardous chemicals, the anticipated increased use of various types of pipes in double containment applications is a certainty. As such, the structural design of these fittings and method of installing and repair of double containment pipes containing such fittings is necessary.
The present invention provides novel double containment fittings for forming a double containment pipe joint. The products that have been developed to date have not completely addressed some of the unique problems that arise when a pipe is placed within another pipe.
One such problem is the ease of maintaining, modifying or repairing such systems. To date, all above ground pressure systems have been installed without the ability to assemble or disassemble the piping or components in modular sections. Therefore, if a repair is to be made, the original system manufacturer/contractor must be called in to facilitate the repair. In many designs, a repair of a certain section would not be feasible or even possible due to the location of the system with respect to adjacent equipment or building parts. What would be very helpful to most facility owners is the ability to have a modular system that is capable of being readily disassembled. A system capable of being readily disassembled would give the facility engineer the ability to have the section requiring modification or repair sent to a shop-type environment readily suitable and equipped to facilitate the change or repair.
Another problem is the ability to provide a means of internally anchoring the inner piping of a double containment piping system in a system with homogeneous inner-outer materials in a cost effective manner. While there have been other parts developed for this purpose (e.g., "Dogbone"-fitting), none have been designed to be able to withstand combined effects of pressure, thermal stress, pipe axial loads, pipe bending, pipe torsion, external soil loads, etc., acting concurrently on the part, due to both inner and outer conditions, under normal operating conditions. The previous parts designed are subject to possible premature failure due to the development of high residual stresses under relatively mild and commonly expected design conditions.
Still another problem is the ability to provide an efficient means of termination of double containment pipes in homogeneous systems, with all of the same considerations imposed in the above paragraph. Whenever a transition from double containment to single containment occurs, the parts will act as a point of rigid interconnection and as a point of anchorage.
Another such problem has to do with the ability of a system to withstand the effects of inner and outer piping and components that are subjected to different amounts of thermal expansion and contraction. It is the norm, rather than the exception, that the inner and outer pipes of a pressure rated double containment piping system are subjected to different amounts of thermal expansion. This situation may arise in several different ways. The most common way involves the situation whereby a hot fluid is transported through the inner pipe. Under this circumstance, the external environment (external that is to the outside diameter of the secondary containment piping) is normally at a lower temperature than the hot fluid. Since there is either an insulating dead air space between the two pipes, or other insulating material, the inner piping temperature becomes close to that of the fluid, while the outside piping remains closer to that of the external ambient environment. Therefore, the materials normally grow to different lengths due to their being at different temperatures. When this does occur, there are thermal strains that are imposed on interconnecting parts and on parts such as interstitial supports that create a contact point between the inner and outer piping. The most obvious place where there is a problem potential is at the interconnecting points. However, any place where loads can be transmitted back and forth between the two pipe systems can result in a problem. Usually, all interconnecting parts designed for this situation are constructed of a singular material, with such a design that the residual stresses and subsequent strain on the materials can lead to a failure of such parts. These existing parts then become the point in the overall system at which failure is most likely to occur, due to either mechanical failure resulting from exceeding the allowable stress of the material, or due to corrosion stress cracking, or other stress-induced phenomena. This is compounded by the fact that the components are truly single containment at that point, without having a containment area to prevent the hazardous fluids from reaching the environment. Therefore, exactly where it would be the least desirable location for failure to occur is the most likely place for failure to occur. Additionally, a double containment piping system that is constructed of an interconnecting part as described above, actually increases the chance that failure to the external environment can occur as compared to a singular containment piping system designed with proper treatment of the thermal expansion of its components.
While certain of these design criteria may have been incorporated in the prior art, none meet all. For example, the fittings illustrated in U.S. Pat. Nos. 4,786,088 and 4,930,544 (FIGS. 8 to 10), commonly referred to as "Dogbone" fittings, may be used to isolate sections of the pipe in the event of a leak, and may be used to enable both pipes to be coupled to each other and to function as a termination fitting. However, due to combined excessive stress on the material of the joint when thermal expansion or internal or annular pressures are encountered, failure of the joint is more likely to occur due to a lack of attention to stress risers and to a lack of reinforcement in areas that are highly susceptible to stress.